This invention relates to a design for two-stroke engines that reduces the exhaust pollutants, short circuit loss of fresh charge, promote lean burning ability of the engine, and stratify the charge and thus improve the overall emission and efficiency characteristics of the two-stroke type internal combustion engine. Two-stroke engines are known for their simplicity and high specific output. However, they have drawbacks of poor emissions and efficiency characteristics. It is estimated that fuel consumption and hydrocarbon emissions of two-stroke engines are 1.5 to 2.0 times and 10-20 times that of equivalent four-stroke engines respectively. Some small utility engines produce up to 50 times the pollution of trucks, per horsepower per hour. High unburned hydrocarbon emissions arise because in a carburetted two-stroke engine the scavenging process is carried out by the fresh mixture of air and fuel. Some of the air-fuel mixture mixes with the residual exhaust gas as it scavenges the cylinder and a small fraction of the charge is lost due to short circuiting. The net effect is that 25-40% of the charge may be wasted resulting in high fuel consumption and high levels of unburned hydrocarbons. In order to eliminate the problem, it has been proposed by Nagesh S. Mavinahally, through his patent application Ser. No. 08/120,545 and disclosure documents 338055 and 336827 to additionally provide a buffer gas in the transfer passages/ducts to enter the cylinder during the scavenging process ahead of fresh charge to prevent or minimize short circuit loss of fresh charge into the atmosphere. It has been demonstrated by him and others that the concept of providing buffer gas can minimize the loss of fresh charge. The buffer gas enters the upper part of the transfer duct displacing the fresh charge remaining from the previous cycle into the crankcase chamber. The amount of buffer gas that is retained in the duct is dependent upon the duct volume and the gas allowed to flow into the ducts. But the volume of the duct depends on the length of the duct from the port to the crankcase chamber. However, it is experienced that the amount of buffer gas introduced into the transfer duct is to be regulated and is dependent on the engine load. With a fixed volume of the transfer duct, as the amount of buffer gas flow is increased to an amount more than the volume of the transfer duct the excess buffer gas simply mixes with the fresh charge diluting it in the crankcase chamber. It is experienced that excessive dilution is detrimental to engine performance at lower loads, which may increase the emission and deteriorate efficiency. It is understandable that more amount of buffer gas is required at higher loads due to larger percentage of short circuit loss and converse is true at lighter loads. It is therefore important to have an optimum amount of buffer gas in the transfer duct.
The amount of buffer gas that remain in situ between the live crankcase content of live mixture and the closed transfer/auxiliary scavenging port to the cylinder is an important factor. It is dependent on the volume of the transfer passage. With its given fixed volume in more conventional engines, increasing the introduction of buffer gas does not guarantee its effectiveness, unless it remains in the transfer duct. An amount greater than the duct's volume simply mixes with the live mixture diluting it in the crankcase chamber.